Nitrosoureas including BCNU, CCNU, methyl-CCNU and streptozotocin are of clinical value in the treatment of a broad spectrum of neoplasms, including CNS diseases, leukemia, Hodgkin's disease and others. They have the distinct advantage of penetration of the blood-brain barrier. Present evidence favors the nucleic acids as one of the principal target sites although the mode of action is not understood. Certain suggested plausible breakdown products of the nitrosoureas will be prepared. The chemical interaction of these compounds together with the parent agents with nucleic acids will be examined in vitro and in vivo. The study employs sensitive and versatile ethidium fluorescence assays which permit the direct detection and quantitation of e.g. alkylation, cross-linking and depurination of DNA by nitrosoureas and their metabolites as well as the base specificity. The mechanism of the reported single strand scission of DNA by nitroso compounds and its bearing on carcinogenesis, mutagenesis and antitumor action will be made. Attempts will be made to correlate extent, type, and rate of chemical modifications of DNA with the antitumor properties of nitrosoureas as has been done successfully for other agents. The understanding thus gained on the mode of action has permitted the rational design of an active antileukemic agent.